1. Field of the Invention
This invention relates to stabilized stainless steels in general and particularly to stabilized ferritic stainless steels suitable for long term use in the service environments frequently found, for example, in moisture separator reheater and feedwater preheater equipment in the steam supply systems of commercial electrical power generating stations.
2. Description of the Prior Art
In the metallurgical arts, stainless steels are those alloys of iron which contain sufficient amounts of alloying elements, principally chromium, to impart corrosion and scale resistance. It is known in the art that at least about 12 weight percent chromium is required to impart the levels of corrosion and scale resistance commonly attributed to stainless steels in aqueous media. The corrosion and scale resistance imparted by chromium is derived from its ability to form a protective, or passivating, film.
Under certain circumstances, however, it is possible to lose the beneficial effects of chromium through a phenomenon frequently referred to as sensitization. A sensitized stainless steel is one which is susceptible to intergranular corrosion (IGC) or intergranular stress corrosion cracking (IGSCC) as a result of the precipitation of chromium-rich carbides at the grain boundaries. With the formation of the precipitates there is a concomitant depletion of chromium below the level required for corrosion resistance in areas adjacent to the grain boundary. This description of the mechanism by which stainless steels are sensitized is referred to in the art as the grain boundary chromium depletion model. Sensitization has been most extensively studied and the bulk of the prior art literature relates to the austenitic class of stainless steels.
Susceptibility to sensitization is a function of the chemistry of the alloy; the prior physical condition including, for example, the extent of any cold deformation; and the nature of sites available for the precipitation of chromium-rich carbides. Sensitization is also a kinetic phenomenon. Therefore, sensitization is also a function of the techniques used in its assessment and can be influenced by the thermo-mechanical history developed during processing and fabrication. For example, a stainless steel heated to an elevated temperature, as during a welding operation, and cooled slowly may exhibit sensitization whereas the same stainless steel heated in the same manner but cooled very rapidly may not.
One technique for preventing sensitization is to reduce the carbon content to extremely low levels (typically less than about 0.030%). Low carbon levels minimize the amount of carbon available for carbide formation and, therefore, the extent of formation of chromium-rich carbides. For some applications, however, such as where high strength is required, a decreased carbon content is not desirable.
Another means for avoiding sensitization is to add other alloying elements, known as stabilizers, such as niobium and titanium, which have stronger carbide-forming tendencies than chromium. These elements rather than the chromium form carbides thus permitting the matrix to retain the corrosion inhibiting chromium. However, the use of niobium or titanium has been shown not to be a panacea as it is possible to sensitize these so-called stabilized austenitic stainless steels through improper heat treatments or fabrication techniques. Also, excessive amounts of these elements may embrittle the stainless steel.
The phenomenon of sensitization of ferritic stainless steels was first reported in 1933 by Houdremont and Shafmeister (Archiv. fur das Eisenhuttenwesen, 7, p. 187, 1933). Since that time, relatively few investigations have been conducted on the sensitization of ferritic stailness steels compared to the vast number of papers published on the sensitization of austenitic stainless steels. The apparent lack of interest in the sensitization behavior of ferritic stainless steels in the past was chiefly due to their low toughness and, therefore, limited usefulness as materials of construction. Then, in 1950, Binder and Spendelow reported in the Transactions of the ASTM (43, p. 759, 1950) that the toughness of ferritic stainless steels could be greatly improved by the reduction of the interstitials carbon and nitrogen. The advent of special melting practices, e.g., argon-oxygen decarburization (AOD), vacuum-oxygen decarburization (VOD), and electron beam melting, made available low interstitial ferritic stainless steels of excellent toughness. The application of ferritic stainless steels to structural components was, therefore, no longer necessarily precluded by low toughness and in many applications the use of ferritic stainless steels is now largely regulated by corrosion resistance. Consequently, an understanding of the mechanisms of sensitization of ferritic stainless steels, particularly the stabilized ferritic stainless steels, become of importance.
The earlier work was conducted on unstabilized ferritic stainless steels and employed the techniques pioneered during studies of the austenitic stainless steels. The prior art investigators have generally concluded that the grain boundary chromium depletion model adequately explains the sensitization of unstabilized ferritic stainless steels, as it did for the austenitic stainless steels, although the kinetics for the two types of steel are generally different.
If grain boundary chromium depletion is responsible for the sensitization of ferritic stainless steels, then, as in the case of austenitic stainless steels, the addition of titanium and niobium should inhibit sensitization. That general premise is true, however, there exist differences of opinion among the experts relative to the amount of titanium required for the stabilization of ferritic stainless steels. Further, there are some references extant in the literature which stand for the proposition that excessive additions of stabilizers, particularly titanium, inherently result in mechanical embrittlement particularly when thick, i.e., greater than about 0.254 cm (0.100 in), sections are involved. Work in this area is sparse and, on the one hand, tends to be limited to specific alloys and, as the work of Demo (Met. Trans., 5, 2253, 1974), not readily transferable to alloys of other compositions. On the other hand, the work of Abo et al. ("Stainless Steels '77", Climax Molybdenum Company, 1977) stands for the broad premise that titanium additions should be as low as possible since even small amounts of titanium raise the ductile-to-brittle transition temperature and further additions raise that transition temperature even further to an asymptotic maximum.
As materials of construction, the ferritic stainless steels are prime candidates for replacing conventional materials now found in heat exchangers such as moisture separator reheaters (MSR) and feedwater preheaters (FWP) frequently found in the steam supply systems of fossil-fired and nuclear fueled commercial electrical power generating stations.
Illustrative of the applications and present materials of construction are the moisture separator reheater units. These units are located between the high pressure turbine and the low pressure turbine of most nuclear steam generating units. As its name suggests, an MSR accepts the exhaust steam from the high pressure turbine, separates out the moisture, reheats the steam and directs it toward the low pressure turbine. The moisture separation is achieved by passing the steam over a set of chevron plates onto which the condensate collects. The steam reheat is typically accomplished by passing a first medium, i.e., high temperature pressurized steam, through enclosing members such as thin-walled tubes. These tubes are generally arranged in closely spaced arrays and frequently are finned on the outside to facilitate heat transfer. The second medium, i.e., the exhaust steam to be heated, is passed through gaps in the array. Heat passes through the tube walls from the high temperature steam to the low temperature steam. The service conditions thus encompass exposure to temperatures on the order of about 300.degree. C. and chloride and hydroxide ions, from impurities in the water, which are capable of causing stress corrosion cracking. Welding is extensively used in the manufacture of the tubes. The tubes are frequently formed from strip material formed into tubes, seam welded longitudinally, and finned; adjacent sections of the tubes are butt welded, and the tubes are terminated by welding into tube sheets.
Copper-containing low-alloy steel is one material presently employed as an MSR tubing material. Unfortunately, because of its low corrosion resistance, a rust-film can form on the surface of the finned low-alloy steel tubes. This rust film can bridge the gap between adjacent fins (a phenomenon referred to as "rust bridging") thereby reducing the heat transfer between the steam flowing inside and outside of the tube.